The increasing amount of data available in digital format requires developing appropriate data storage systems. In many cases, the amount of data to be stored exceeds the capacity of a single disk drive. Furthermore, the reliability of a single drive may not be sufficient for a particular application. This motivates the development of redundant disk arrays such as, for example, a Redundant Array of Independent Disks or RAID. The size of such arrays may vary from a few disks to a few thousand disks. Such arrays employ some kind of error/erasure correcting codes, which provide the redundancy needed to recover the data if one or more of the disks fail.
However, in many cases, a disk remains operational, while only a few blocks on the disk become unreadable. These unreadable blocks are typically discovered when some other disk fails, and the data on remaining disks is being read in order to re-calculate the data on the failed disk. The need to be able to cope with such latent media errors is a main rationale for developing RAID architectures with many parity disks (e.g., RAID-7.3). However, the need to allocate additional parity disks significantly increases the cost of data storage systems.